A Basin-Scale Modelling Methodology to Link the Poro-Mechanics of a Source-Rock Formation and the Tectonic Faulting in Fold-and-Thrust Belts

Abstract

Previous kinematics of structural evolution of the Chartreuse Fold-and-Thrust Belt (FTB) have shown that the tectonic structures’ development is concomitant with hydrocarbon fluids generation within the décollement level, which suggest a link between the two processes.

Combined basin and petroleum system analysis of fluids pressure evolution and geomechanical modelling of tectonic faulting were performed to study and quantify the conditions necessary to activate and propagate a décollement within sedimentary overburden of fold-and-thrust belts. Here the main case study referred to is the Chartreuse FTB and its Liassic source-rock decollement during the main shortening phase.

Using an analogue of the western alpine Toarcian formation found in south Albania, a laboratory characterization of centimeter-scale rock samples was performed. The petrophysical rock properties (porosity and permeability), the mechanical rock parameters (elasticity, rock strength, friction coefficient) and the geochemistry of the kerogen (TOC) were determined.

These measurements where instrumental to set up some assumptions for the initial state of the geological formation before modeling tectonic loading and thrust faults activation. The measured mineralogy and TOC of the Albanian Posidonian Shale was used as proxy and these results reveal a centimeter-scale and a meter-scale mechanical stratigraphy for the source-rock formation. Pore pressure evolution and source-rock rheology cannot be considered a simple and uniform set-up for the source-rock.

The basin-scale numerical models show that fluids overpressure (OVP) within a décollement layer is strongly dependent on the rate and magnitude of the source-rock burial. Interestingly, the models point out that this burial must be calibrated both before and during the folding phase, because the pre-shortening properties of the décollement, acquired since deposition, impacts its activation. During the main shortening phase and neglecting tectonic stresses, fluids OVP within the Toarcian décollement only becomes significant when considering both a low permeability source-rock and a high HC generation rate. Still, in our models, the distribution of the OVP remains strongly correlated with the burial, and only in the most extreme case significant propagation toward the foreland is observed.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018